The present invention relates to emitter-switched transistor structures and, more specifically, to emitter-switched insulated gate bipolar transistor (IGBT) and emitter-switched thyristor structures.
Emitter-switching is directed to a circuit configuration in which a low-voltage MOS power transistor cuts off the emitter current of a high-voltage bipolar power transistor. Such a structure is disclosed, for example, in U.S. Pat. No. 5,118,635 which is incorporated herein by reference.
The structure of the prior art emitter-switched transistor of U.S. Pat. No. 5,118,635 is shown in FIG. 1. The high voltage bipolar transistor 20 is formed by the bottom layers of the chip. Specifically, an N- conductivity epitaxial layer 2, which serves as the collector of the bipolar transistor, is grown on an N+ type substrate 1. A P+ region 3, which constitutes the base of the bipolar transistor, is formed in layer 2 and extends to the upper surface of the chip at peripheral regions 8 to allow for external connection. A buried N+ region 4, the emitter of the bipolar transistor, is provided over P+ region 3.
The low voltage vertical MOS power transistor (MOSFET) 22, which cuts off the current supplied to the emitter of the high-voltage bipolar transistor, is provided at the top surface of the chip. The MOSFET consists of a N type drain region 5 disposed over and contacting buried N+ region 4, P body regions 6, N+ source regions 7, and insulated gate 9. Metal contacts 10, 11 and 14 are provided for contact with regions 6 and 7, 8 and substrate 1, respectively.
FIG. 2 shows the equivalent circuit of the prior art structure of FIG. 1. Here, the prior art emitter-switched transistor structure has four terminals, with the base 11 of the power bipolar transistor 20 and the gate 13 of the power MOSFET transistor 22 being controlled independently of one another.
It would be advantageous to provide an emitter-switched bipolar transistor in a three-terminal configuration. It would be further advantageous to provide a conductivity modulated three-terminal emitter-switched bipolar transistor with reduced on-resistance. It would also be advantageous to provide an emitter-switched thyristor configuration with the above-noted attributes. Finally, it would be advantageous to provide an emitter-switched bidirectional power transistor.